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Microbial Fuel
Cells (MFCs)
Spring 2013
Iridescent Lesson Plan
Microbial fuel cells (MFCs) can be used as a tool for multidisciplinary
learning for students of all ages. This lesson plan is designed to teach
students, ages 9-14, the basic concepts of electricity: voltage, current,
and resistance. Through hands-on experimentation, students will see
theory in action in a MFC.
This lesson plan was prepared by Andrew Leader (EE ’13) and Stefania
Samojlik (ME ’13) from the Cooper Union.
Microbial Fuel Cells
Microbial Fuel Cells
IRIDESCENT LESSON PLAN
OBJECTIVES
1. Introduce fundamental concepts of electricity: voltage, current, resistance
2. Apply knowledge to understand the basics of microbial fuel cells (MFCs)
3. Experiment with MFC kit
A QUICK REVIEW OF ELECTRICAL CONCEPTS
Voltage
Voltage is the electric potential difference between two points. This potential difference drives the motion of
an electrically charged particle.
Voltage is measured in SI units of volts (V).
Current
Current is the flow of electric charge between two points in a conductor. The direction of conventional current
follows the flow of positive charge. Thus, the direction of conventional current is the opposite of the flow of
negative charge.
Current is measured in SI units of amperes (A).
Resistance
Resistance is the opposition to current. It is also the ratio between voltage and current. This relationship is
called Ohm’s Law which states that V=IR where V is voltage, I is current, and R is resistance.
Resistance is measured in SI units of ohms (Ω).
INTRODUCTION TO MFCS
In 1910, M. C. Potter first observed the ability of E. coli to produce electricity [1]. Ever since, scientists have
studied the ability of microbes to produce electric potentials in depth, and have incorporated this phenomenon
into the design of microbial fuel cells (MFCs), which take advantage of natural biological processes in the
microbes to catalyze the conversion of chemical energy in organic fuels into electrical energy. Recently, the
search for alternative forms of energy has brought renewed interest to MFCs.
Benefits of MFCs
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Microbial Fuel Cells
MFCs offer many potential advantages over other means of localized power generation. In general, since
fuel cells do not use combustion, their efficiencies are not limited by the Carnot cycle. The microorganisms in
MFCs can derive energy from many different types of fuels [1], making them convenient for situations where
refined fuels are not available. While the substrate molecules are oxidized via microbe metabolism as
opposed to combustion, there are no harmful, partially oxidized byproducts such as carbon monoxide.
Although different types of MFCs have been designed for various operating conditions, MFCs are generally
operated at room temperature and neutral pH, so they can be employed where maintaining harsh conditions
is impractical or undesirable, unlike many other types of fuel cells.
Applications
Many potential applications for microbial fuel cells have been described, and some have been implemented
with varying degrees of success. Among these applications, methods of wastewater treatment have proven the
most successful [1–3], and a small industry of startup companies has already begun efforts to scale up MFC
wastewater management processes to municipal levels. MFCs have also been used to power networks of lowpower sensors [1, 4]. In at least one case [3], a robot was powered by an MFC. This so-called gastrobot was
fueled by sugar cubes fed to the MFC anode compartment.
Other attractive applications which have been proposed include MFCs for off-grid power, particularly for use
in poor, rural communities. For example, researchers in India have developed an MFC for less than $1, made
from commercially available, locally produced earthen pots, for use as a wastewater treatment and local
power generation device [5]. The MFC gave a maximum power output of 70 W/m3, and a Coulombic
efficiency of 64.5%.
Groups have also begun proof-of-concept studies on the use of MFCs in vivo to power implanted medical
devices such as pacemakers [6, 7]. While many types of bacteria can also produce useful chemicals such as
methane and hydrogen, the use of MFCs as sources of these chemicals has also been proposed [2].
Limitations, and how they may be addressed by students
There are still many limitations that impede MFC development for widespread, practical use. Many materials
used in state-of-the-art MFCs can be costly, such as custom electrodes and proton exchange membranes.
However, some researchers have shown that cheaper alternatives exist, depending on the applications [4, 5].
The types of microbes capable of generating a potential in an MFC are extremely diverse, and each has its
own electrical and metabolic properties; full understanding of the electricity generating processes for most
microbial species is far away, and optimizing these processes is even further [8].
Most research has been focused on experimenting with different bacteria, mediator molecules, and electrode
materials instead of looking directly to applications. A lack of standards between labs hinders their ability to
compare results. While the highest power densities are around 4 W/m2 [9], this remains too low to be a cost
effective means of power generation for most applications. Although there has been success for some
applications in the lab, no one has demonstrated scalability in any practical sense.
MFCs have already been used as fun, educational tools for young students and scientists. MFCs make good
teaching tools because they can be approached from many angles; as an extremely interdisciplinary subject,
the MFC provides strong lessons about many subjects and how they interface in one system.
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Microbial Fuel Cells
HOW MFCS WORK
Summary of MFC Operation
The operation of the proposed MFC is depicted in Figure 1. In overview, microbes in MFCs break down fuel
molecules via their natural catabolic chemical reaction pathways [1, 3]. While some of the energy released
from these reactions is used by the organism to power its own functions, some can also be harnessed by
introducing electron-mediators into the culture. These electron-mediators reduce chemicals in the catabolic
pathways, then diffuse from the cells and deliver electrons to the anode. Meanwhile, protons, another
catabolic product, get released and diffuse through a cation specific exchange membrane to react with
electrons and oxygen to form water at the cathode. This separation of charge gives rise to an electric
potential across the anode and cathode, which is used to power a load.
In Figure 1, the specific sequence of events corresponding to the numbers (1-7) goes as follows:
1. The microbe takes in fuel (glucose) and the mediator molecule (methylene blue, MB), and digests
the glucose. During this process, electrons (e-) bind to the mediator molecule.
2. The microbe excretes the negatively-charged MB and protons (H+) into the surrounding fluid.
3. Electrons are transferred from MB to the anode.
4. MB is recycled.
5. Electrons travel from the anode to the cathode across a load.
6. Meanwhile, protons diffuse across the cation exchange membrane to the cathode chamber.
7. Protons, oxygen, electrons and ferricyanide ([Fe(CN)6]3-) react to form water. The ferricyanide, a
catalyst, is recycled.
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Microbial Fuel Cells
FIGURE 1: DIAGRAM OF MFC PROCESSES
The following subsections describe each of these steps in closer detail. The relevant project work uses
Saccharomyces cerevisiae, a common species of yeast, as the microbe, methylene blue as the electron mediator,
potassium ferricyanide as a cathode catalyst, and glucose as the fuel. Therefore, MFC operation with these
components will be explicitly outlined as an example.
MFCs in closer detail
Catabolism and electron transfer
The biological breakdown of complex chemicals in order to release energy is referred to as catabolism.
During these reactions, the energy-rich fuel molecules are oxidized to simpler molecules. Oxygen has a high
reduction potential of 0.82 V, so aerobic reactions, or those which require oxygen, tend to be highly favored
during catabolism over anaerobic reactions, which can proceed without oxygen [11].
In order for power to be transferred to the fuel cell circuit, an intermediate oxidizing agent must be
substituted in the anode chamber for oxygen. While oxygen acts as a final oxidizing agent in the cathode
chamber, inserting an intermediate step into the overall oxidation of the fuel allows for some of the redox
potential to be developed across the MFC electrodes. Many chemicals are capable of serving as this redox
intermediate, and some are naturally produced by different kinds of bacteria when oxygen is not present
[11]. Fig. 3 shows some of these chemicals in their oxidized and reduced forms, and where they fall in order
of reduction potential; such a list of redox couples, ordered by reduction potential, is often referred to as a
redox tower, and is analogous to vertical steps on a potential energy graph.
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Microbial Fuel Cells
Canonically, the example fuel molecule most often discussed is glucose;
glucose is a highly prevalent and important molecule, and many larger
molecules, such as starches, are first broken down to glucose before
they are degraded further. While catabolism of other types of food
molecules may follow slightly different pathways, most catabolic
pathways have highly conserved motifs. Conveniently, glucose also
serves as the fuel molecule used in the relevant project work, which will
be discussed later.
In S. cerevisiae, our example microbe, anaerobic catabolism of glucose
proceeds according to the pathway shown in Fig. 4.
FIGURE 2: A REDOX TOWER LISTS REDOX
COUPLES
Before catabolism begins, glucose diffuses into the cell. This transport
is enabled by passive transport proteins, which specifically pass
glucose through the cell membrane with a set permeability. In the first
step of glucose catabolism, each glucose molecule undergoes
glycolysis, or breakdown to two molecules of pyruvate while two
molecules of adenosine diphosphate (ADP) are converted to two
molecules of adenosine triphosphate (ATP), an energy storage
molecule, and two molecules of nicotinamide adenine dinucleotide
(NAD+) are reduced to two molecules of NADH (simply referred to as
reduced nicotinamide adenine dinucleotide), a different energy
storage molecule. One molecule of carbon dioxide then separates
from each molecule of pyruvate, forming acetaldehyde. While ATP is
recycled to ADP when it is used to power the organism’s other
functions, NAD+ is also required for glycolysis and must be recycled by
a redox reaction. Therefore, NADH is oxidized by acetaldehyde to
NAD+, and acetaldehyde is reduced to ethanol.
FIGURE 3: A SIMPLIFIED FLOW CHART OF ANAEROBIC GLUCOSE CATABOLISM IN S. CEREVISIAE VIA FERMENTATION [12].
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When methylene blue is introduced to the S. Cerevisiae culture, it plays the role of NADH oxidizer, obtaining
two electrons as NADH is converted to NAD+. Although it seems that this leaves acetaldehyde to accumulate,
these reactions are very tightly regulated by other metabolic players, particularly with respect to
NAD+/NADH [13]. Since methylene blue presence inhibits the reduction of acetaldehyde to ethanol, pyruvate
is processed along an alternate metabolic pathway. For example, pyruvate can react to form any of a large
number of other molecules, such as alanine, aspartate, isoleucine, phenylpyruvate, or valine. The degree and
timescale of waste molecule buildup, as well as the identity of any waste molecules, is an ongoing topic of
research.
Electron transfer
Much current research on MFCs focuses on determining more effective means of transferring electrons from
within microbes to the anode [1–4, 8, 14]. While S. Cerevisiae requires the addition of an electron-mediator
molecule, certain strains of bacteria called exoelectrogens are capable of attaching directly to the anode and
depositing electrons on the anode directly. While exoelectrogens have been shown to produce much higher
power densities in MFCs than non-exoelectrogenic microbes, we limit our discussion to the simple case of S.
Cerevisiae due to safety and cost restrictions for the applications to be discussed.
While the exact mechanisms of electron delivery by the reduced form of methylene blue onto different anode
materials are unknown, the reactions incur measurable activation loss. While this loss is important, it is heavily
outweighed by ohmic loss [13].
Proton exchange and cathode reactions
Protons, another catabolic product, are secreted by the microbes during MFC operation. The protons diffuse
across a cation-specific membrane to the cathode chamber, where they react with oxygen and electrons to
form water, completing the circuit.
Different types of materials may be used for the cation exchange membrane, though the most successful ones
tend to be porous polymers with complex structure and charged functional groups, such as sulfate. Positive
ions diffuse through the pores, finding local potential energy minima at the charged functional groups. In
order to achieve efficient transport through the membrane, it must be well hydrated beforehand. Often,
potassium ferricyanide is added to the cathode as a catalyst to prevent the partial reduction of oxygen to
peroxide, which results in dramatic voltage loss [13].
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LESSON STRUCTURE
Prep & Setup
Andrew and Stefania will come and setup the experiments an hour before class begins.
Time: 1 hr
Introduction of electrical concepts – lecture & experimentation
Students will be introduced to concepts such as voltage, current, and resistance. Students will also experiment
with batteries, resistors and LEDs and observe the effect of changing resistance on the brightness of an LED.
Time: 40 mins
Discussion of MFCs and their applications – lecture
Students will learn about how MFCs work and how this technology can be applied to “green” waste water
treatment. The goals and procedure for MFC experiment will also be explained.
Time: 20 mins
MFC experimentation
Students will assemble and set up an MFC. They will measure voltage across various resistors. Students will be
supplied with handouts to fill out with their results.
Time: 1 hr 30 mins
Discussion of results
Students will discuss what they observed during the experiments and compare their results.
Time: 20 mins
Clean Up
Andrew and Stefania will clean up and disassemble the MFCs.
Time: 30 mins
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Microbial Fuel Cells
MATERIALS (FOR 1 MFC)
MFC Construction:
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2x Acrylic End Plates
2x Polypropylene Chambers
4x Rubber Gaskets
2x Graphite Electrodes
1x Cation Exchange Membrane
7.0 pH 0.1M Phosphate Buffer



10.71 g Potassium Phosphate Monobasic
5.24 g Potassium Phosphate Dibasic
1100 mL Distilled Water
Anode Solution:
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40 mL 7.0 pH 0.1mM Phosphate Buffer
0.13 g Methylene Blue
3.4 g Yeast
7.20 g Glucose
Cathode Solution:


40 mL 7.0 pH 0.1mM Phosphate Buffer
0.13 g Potassium Ferricyanide
8% Salt Solution


4 g Sodium Chloride
50 mL Distilled Water
Equipment for Setup and Experimentation
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1x Multimeter
5x Resistors
1x LED
1x Breadboard
1x Funnel
Glassware
TEACHER PREPARATION
1. Prepare 8% NaCl solution, and soak cation exchange membrane in salt solution for at least 24 hours.
2. Prepare phosphate buffer, anode solution and cathode solution.
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Microbial Fuel Cells
STUDENT PROCEDURE
1. Put on latex gloves and safety goggles.
2. Assemble MFC with pre-soaked cation exchange membrane and graphite electrodes as shown below
3.
4.
5.
6.
Pour cathode solution into one chamber, then anode solution into the other chamber using a funnel.
Hook up graphite leads to multimeter and observe the voltage.
Hook up graphite leads in series with various resistors and measure the change in voltage.
Record voltage measurements on handout.
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STUDENT HANDOUTS
Introduction to Electrical Concepts
What is electricity?
Electricity exists everywhere around us from lighting in the sky
to the batteries in your calculator to the electrical impulses in
our nerves. This phenomenon can be attributed to small
charged particles interacting with each other and their
surroundings.
“Opposites Attract”
These charged particles can be negatively (-) and positively
(+) charged. One special characteristic of such particles is that
they can be attracted or repelled by each other. The saying
“opposites attract” is useful when remembering how these
particles interact with each other. Negative and positive
charges are attracted to each other, 2 positive charges repel
each other, and 2 negative charges will repel each other.
Voltage, Current, and Resistance
By the laws of physics, when two oppositely charged particles are close together, they will move toward each
other. The flow or movement of these charged particles is called current. Another electrical quantity, voltage,
is used to measure the “electric potential energy” between two points. The concept of voltage can be quite
confusing, but it is similar to a person on a slide. The person on the top of the slide is like a charged particle.
The voltage can be compared to the height difference between the top and bottom of the slide. Once the
person starts moving, he represents the current. If the slide is higher, the person will reach a faster speed at
the bottom of the slide. The same is true of a charged particle. If the voltage is larger, the charged particle
will move faster. In other words, if the voltage is higher, the current is higher. This relationship between
voltage and current is very important in studying electricity, and it is called Ohm’s Law. There is one more
property that plays a part in Ohm’s Law, called resistance. Resistance is the opposition to the flow of
charged particles. You can imagine that how fast you go down a slide depends on the type of clothes you
wear. For example, to slow down on a slide, you can use your shoes as “brakes”. In this situation, your shoes
are like the resistance that decreases the current. So if there is a large resistance, the current will be much
smaller.
Procedure
Introduction to Microbial Fuel Cells
A microbial fuel cell (or MFC) is very similar to a battery which provides a voltage between the two points. If
these two points are connected together through a wire, they can provide a path for current to flow. However,
MFCs are much different than batteries because they use small organisms like yeast to generate a voltage.
Parts of an MFC
The MFC has several important parts.
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Microbial Fuel Cells
<insert diagram>
Anode Chamber
Cathode Chamber
Electrodes
Cation exchange membrane
How do they work?
How are they used?
Green energy!
Waste water treatment!
MFC Procedure and Results Handout
Lab Objective
Materials
Procedure
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Results Table
Trial
1
2
3
4
5
6
Questions
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Resistance (Ω)
Voltage (V)
Microbial Fuel Cells
REFERENCES
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Microbial Fuel Cells
APPENDIX – MSDS SHEETS
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